Printed wiring boards such as a flexible printed circuitry (hereinafter abbreviated as FPC) have been used in small portable electronic devices such as smartphones and tablet PCs for reasons of the ease of wiring and the lightweight properties that the printed wiring boards have. For such FPCs, two-layered flexible boards that are formed by laminating a copper foil on a polyimide-base resin board and integrating them with an adhesive agent or thermocompression are used. Another type of two-layered flexible boards that are formed by forming a resin layer of polyimide resin as an example on a copper foil is also used. The method of forming the resin layer is not specifically limited. However, a polyimide-base resin layer can be formed by applying a mixture containing polyamic acid obtained from addition polymerization of aromatic diamines and aromatic dianhydride in a solution state to a copper foil, drying the applied mixture, forming a polyamic acid layer as a polyimide precursor layer, imidizing the polyamic acid layer by heating the layer to 300-400° C. under an inert atmosphere of nitrogen etc.
Conventionally, a subtractive method has been used to form a desired wiring pattern on an FPC. In the subtractive method, a resist layer is provided on a place to be wiring on a copper foil that is adhered to an insulating resin layer of the two-layered flexible board. The resist layer is exposed to light and partially removed by development, and a copper foil layer exposed from the removed portion is removed by etching with a solution such as a copper chloride solution and a ferric chloride solution. Finally, by peeling and removing the resist layer, a wiring board can be formed. When a wiring pattern is formed with this subtractive method, a cross section of the wiring portion tends to have a trapezoidal shape that spreads toward the bottom. In order to obtain electrical insulation properties, a sufficient width of wiring/space needs to be obtained, and for that reason, there have been limitations to fine-pitch wiring patterns with this method.
Meanwhile, as a method to achieve fine-pitch wiring such as wiring/space (L/S)=20/20 μm or 15/15 μm, which is considered to be difficult with the subtractive method, a semi-additive method has been proposed. In the semi-additive method, a resist layer is provided on a copper foil adhered to an insulating resin layer of the above two-layered flexible board. Next, the resist layer is exposed to light and developed so that the resist on a portion in which a circuit is formed is peeled and removed, and copper plating is performed on the copper foil layer that is exposed from the removed portion. After obtaining a desired thickness of copper layer by the copper plating, the remaining resist layer is peeled and removed, and the shape of the circuit is formed. Next, a copper foil present in a bottom portion between circuits is dissolved and removed by flash etching etc., and a wiring board can be formed.
In recent years, with further reduction in weight, size and thickness of electronic devices, demands on more highly dense wiring have been increasing, and the fine-pitch wiring technique that uses the semi-additive method has becomes increasingly important. In addition, the fine-pitch wiring is also required to have high bending endurance that had been required in flexible wiring boards.
For example, Patent Literature 1 proposes that among electrolytic plating layers, at least one copper sputter layer is provided as an intermediate layer, and as a result, an occurrence of micro-cracking and an occurrence of through-cracking is prevented and characteristics of sliding bends has been significantly improved. Patent Literature 2 proposes that a copper-plated layer has a multilayer structure and a twin particle diameter is less than 5 μm and has good bending endurance tested by MIT.